This application claims the priority of Japanese Patent Application No. 2000-003571 filed on Jan. 12, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image readout apparatus such as copier, facsimile machine, and image scanner, and an image readout method using this apparatus; and, more specifically, to an image readout apparatus comprising a plurality of rows of linear light-receiving devices, and an image readout method for reading out a color or monochrome image by using this image readout apparatus.
2. Description of the Prior Art
An image readout apparatus is an apparatus which illuminates an original with a light source, forms an image of the illuminated original onto a light-receiving device array through an imaging lens, and reads out thus formed image. Recently, there has been an increasing demand for changing the image readout resolution by use of a single image readout apparatus.
Conventionally, when the readout resolution of an image readout apparatus is to be changed, the imaging magnification of the imaging lens has been altered so as to change the size of the image formed on the same light-receiving device array, whereby the image readout resolution is made variable. Alternatively, while a light-receiving device array having a pixel pitch corresponding to the highest resolution in the image readout apparatus is used, processing for electrically reducing the number of pixels, for example, is carried out upon readout at a lower resolution.
In the former case of altering the imaging magnification of the imaging lens so as to change the image readout resolution, it is difficult for the imaging lens to be designed so as to yield favorable performances with a plurality of imaging magnifications, and the apparatus becomes larger and more expensive since the number of its constituent lenses increases. Also, since the conjugate length varies depending on the magnification, mechanical operations are necessary for absorbing this variation, whereby the apparatus becomes more expensive. For making the conjugate length constant, on the other hand, the imaging lens may be constituted by a zoom lens. However, the number of lens elements constituting the imaging lens further increases as a consequence, and the imaging lens enhances its size, whereby the apparatus becomes more expensive. Further, while the distance between the imaging lens and the light-receiving device array must be maintained with a high precision, this distance will be hard to maintain with a high precision if the imaging lens moves.
The latter case of changing the image readout resolution by carrying out electric processing, on the other hand, is structurally simple since it will be sufficient if high-resolution image information is electrically processed upon readout at a low resolution, whereby the cost will not rise greatly. In this case, however, image readout cannot be effected faster at a lower resolution, so that it takes the same amount of time as that at a higher resolution.
It is an object of the present invention to provide an image readout apparatus and image readout method which can change the image readout resolution without requiring complicated electric processing, while the distance between the imaging lens and light-receiving device array is maintained with a high precision.
The present invention provides an image readout apparatus comprising a light source for illuminating an original, a linearly arranged light-receiving device array, and an imaging lens for forming an image of the original illuminated with the light source onto the light-receiving device array;
wherein a plurality of light-receiving device arrays having respective pixel pitches different from each other are arranged at an image-forming surface of the imaging lens.
The resolution ratio between the different pixel pitches may be at least 2.
Three or more kinds of light-receiving device arrays having respective pixel pitches different from each other may be arranged.
Three or more rows of light-receiving device arrays with the same pixel pitch may be arranged, and the device arrays with the same pixel pitch may have sensitivities for respective wavelength regions different from each other.
Preferably, the light-receiving device comprises a linear Charge Coupled Device (CCD) image sensor.
The present invention provides an image readout method in which an original is illuminated with a light source, and an image of the illuminated original is formed onto a linearly arranged light-receiving device array through an imaging lens;
the method comprising the steps of:
arranging a plurality of light-receiving device arrays having respective pixel pitches different from each other; and
reading out an image of the original formed on a light-receiving device array having a smaller pixel pitch upon readout at a higher resolution, and reading out an image of the original formed on a light-receiving device array having a greater pixel pitch upon readout at a lower resolution.
The image readout method may comprise the steps of arranging three or more rows of light-receiving devices having the same pixel pitch, and reading out respective wavelength regions different from each other with the device arrays having the same pixel pitch.